[tor-bugs] #14957 [Tor]: Clients wait for an extra consensus before learning the correct UseGuardFraction value

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#14957: Clients wait for an extra consensus before learning the correct
UseGuardFraction value
-----------------------------------+------------------------------------
 Reporter:  asn                    |          Owner:
     Type:  defect                 |         Status:  new
 Priority:  normal                 |      Milestone:  Tor: 0.2.7.x-final
Component:  Tor                    |        Version:
 Keywords:  tor-client, tor-guard  |  Actual Points:
Parent ID:                         |         Points:
-----------------------------------+------------------------------------
 When clients parse the consensus, they decide whether to register any
 guardfraction information included based on the value of the
 `UseGuardFraction` consensus parameter. The problem is that the decision
 is taken in the middle of consensus parsing,  but in
 `should_apply_guardfraction()` we wrongly use
 `networkstatus_get_param(NULL, "UseGuardFraction",..."` to decide whether
 the parameter is on or off. In that case `networkstatus_get_param()` will
 check the old consensus, and not the one that we are currently parsing and
 is still in unfinished state.

 ticket:9321:comment:52 includes some possible fixes.

 Another possible fix would be to always register guardfraction information
 found in the consensus. But instead check during path building time
 whether we should use it or not. So instead of having
 `should_apply_guardfraction()` we would have
 `should_consider_guardfraction()` or `should_use_guardfraction()`.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/14957>
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